The present invention relates to a wear-resistant shaped hard metal body, and more particularly, to a hard metal body composed of a hard metal body core or substrate which contains a surface coating.
It has long been known that hard metal bodies can be formed from at least one binder or bonding metal of iron, cobalt and nickel and at least one hard metal refractory carbide of at least one of the elements titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum and tungsten. The hard metal body generally is formed by uniting a powdered form of the hard metal carbide by compression with the binding metal, followed by sintering. During the sintering process, the product generally receives its final shape and dimensions and the resulting sintered product is a molded, shaped, hard metal body which often is referred to as a cemented carbide. The hard metal bodies possess great hardness and find wide application in metal turning and cutting tools which are hard enough to permit high turning and cutting speeds in rock or metal.
Increasing demands have been placed on hard metal bodies and there has been a continuing search to provide hard metal bodies having still greater wear resistance. To this end, there has been produced hard metal bodies comprising a core of a shaped, hard metal body formed from a hard metal carbide and bonding metal as described above and a surface coating of a hard material on the core. The surface coating of hard material has been made from such materials as carbides, nitrides, borides and/or oxides, and generally is formed by deposition on the core of the hard metal body during a separate process step. For example, deposition from the gaseous phase according to the chemical vapor deposition (CVD) process is a preferred method for forming a surface coating on a hard metal body. Among the oxides which have been used for forming surface coatings, aluminum oxide and zirconium oxide have been found to be particularly useful as wear-resistant surfaces on the hard metal body core.
In addition to providing an outer layer of a surface coating of hard material on the core of the hard metal body, an inner or intermediate layer has been provided between the core and the outer layer of the surface coating. The main purpose of the inner layer is the equalization of stresses. Metals, such as cobalt, nickel and iron, have proved particularly suitable for this, also precious metals, such as platinum. The inner layers can be applied to the hard metal body by electrodeposition. Inner layers can also be formed by the CVD process or one of the PVD processes.
In one prior art hard metal body containing a surface coating, the hard metal body core contains at least one carbide suitable as a hard material and at least one binder metal and the surface coating is composed of two superimposed layers of which the outer, extremely wear-resistant layer consists of at least one material selected from the group consisting of aluminum oxide and zirconium oxide. In German Offenlegungsschrift No. 2,253,745, published on May 17, 1973, there is disclosed such a shaped body whose inner layer is composed of one or a plurality of carbides and/or nitrides of the elements titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, silicon and boron. These known shaped bodies, however, have the drawback that their outer layer often does not have sufficient adhesion. The problem of suitable adhesion of a surface coating to a core of a hard metal body is especially of concern in tools used for turning and cutting operations because the tools are subjected during this use to high impact stresses and strong alternating thermal stresses which often cause the surface coating to chip off which leads to premature failure of the tools.